The substitution method has a major flaw in it saying that it can be cracked with frequency analysis. Frequency analysis is the analysis of letters which appear frequently in a message or a text. One way to solve this issue is to replace letters with Greek Letters and manipulate it in a way of making it look miscellaneous. Letters could be replaced with different stroked which have been previously agreed on by the sender and receiver.
You could have one stroke meaning ‘A’, a cross meaing ‘B’, two horizontal strokes and a vertical meaning ‘C’, and so on.